1. Field of the Invention
The present invention relates to a titanium alloy material with excellent high-temperature oxidation resistance and corrosion resistance, which is useful as an exhaust system material for a vehicle or motorbike (motorcycle), for example. More specifically, the invention relates to a titanium alloy with improved oxidation resistance at high temperatures and corrosion resistance, while having inherent outstanding strength characteristics and corrosion resistance thereof without impairing economy and workability.
A titanium alloy of the invention can be widely applied to various applications where it might be exposed to oxidizing atmosphere or corrosion environment at high temperatures, with its excellent high-temperature oxidation resistance and corrosion resistance. A case where the titanium alloy is used as the exhaust system material will be mainly explained herein.
2. Description of the Related Art
An exhaust system for a vehicle or motorbike is composed of an exhaust manifold, an exhaust pipe, a catalytic muffler, a pre-muffler, and a silencer (main muffler), which are arranged in this order, starting on an exhaust gas outlet side of an engine. (In the description below, these components are referred to as the “exhaust system” individually and collectively.) Although steel was normally used as a constituting material of these exhaust systems in the prior art, stainless steel having good corrosion resistance has been recently used mainly.
In recent years, however, attention is being given to exhaust systems made of titanium mainly for motorbikes. Since the titanium has various kinds of characteristics which involves lightness in weight, good resistance to vibration fatigue, excellent corrosion resistance, small coefficient of thermal expansion, and excellent resistance to thermal fatigue, as compared to the conventionally normal steel and stainless steel. Thus, the titanium exhaust systems have become increasingly standard equipment not only in most motorbikes for races, but also in motorbikes for mass production.
Most of the titanium exhaust systems, which have already been put to practical use, are made of JIS class 2 pure titanium for industrial applications. The temperature of exhaust gas emitted from an engine of the vehicle or motorbike is normally estimated to be greater than 700° C. In the case of a component whose surface is largely open and exposed to the outside air, such as an exhaust system for a motorbike, since heat is diffused from the surface into the air, the temperature of the exhaust system itself does not become so high, and hence the JIS class 2 pure titanium material can be used without trouble. On the other hand, an exhaust system for the vehicle which is not directly open to the outside air, or even a component of the motorbike disposed at a part which merges with an exhaust pipe or pipes, is likely to be heated to a high temperature. Thus, a titanium alloy having higher heat resistance is desired, as compared to the existing JIS class 2 pure titanium material.
From this viewpoint, some kinds of titanium alloys, such as a Ti-3Al-2.5V alloy, or a Ti-6Al-4V alloy, among the existing titanium alloys, are supposed to be useful materials for the exhaust systems. However, in order to form and assemble of the exhaust system, the material needs to be made into a thin sheet and should have excellent workability. Therefore, the above-mentioned two existing titanium alloys which lack formability cannot meet these requirements.
Thus, the present applicant has previously studied titanium alloys so as to develop a titanium alloy having excellent heat and corrosion resistance, and good formability. As a result, the titanium alloy disclosed in an after-mentioned U.S. Pat. No. 6,531,091 has been developed.
This titanium alloy contains Al in an amount ranging from 0.5% to 2.3% by mass. Preferably, the metallographic structure of the titanium alloy comprises α-phase: 90% by volume or more, and β-phase: 10% by volume or less. Such a titanium alloy is expected to serve as a useful exhaust system material which maintains inherent lightness and corrosion resistance thereof, and enhances heat and oxidation resistance, while improving formability, which is important as the exhaust system material.
While the inventors advance research on titanium alloys, the titanium alloy disclosed in U.S. Pat. No. 6,531,091 has been found to leave room for improvement in corrosion resistance (particularly, crevice corrosion resistance) and high-temperature oxidation resistance. That is, the Al content in this titanium alloy is limited to the above-mentioned range so as to have both of formability and high-temperature strength. Furthermore, the inventors thought that the Al content was preferably not less than about 1.5% in terms of usefulness. However, this level of Al content imposes a limitation on durability when the alloy is exposed to a more severe corrosive environment or a high-temperature oxidizing atmosphere. Thus, further improvement is desired by taking the customer's future demands into consideration.
Moreover, the inventors have proposed another invention disclosed in JP-A No. 55725/2003 as another remedial technique concerning the titanium alloy. This invention is applied to frames for glasses as a major use, which have been recently demanded increasingly. The Al content as disclosed in the document is 0.5 to 2.3%, which is the same level as the above-mentioned content in U.S. Pat. No. 6,531,091, while 4% or less of Ga and 1% or less of Si are included in the alloy without substantially including any β stabilizing elements, thereby improving cold-workability and fatigue characteristics after brazing. In the invention disclosed in this document, the maximum Al content is limited to 2.3%, there by ensuring stable formability. And, addition of Si forms microscopic Ti silicide to inhibit growth of grains after brazing, thereby enhancing the fatigue strength. This titanium alloy, which is applied to the frames for glasses as the major use, is used in an ordinary temperature environment, and belongs to a different type of alloy from the titanium alloy which is used in a severe corrosion environment or high-temperature oxidizing environment like the exhaust system material.